Abstract: One of the key mechanical problems of strengthening RC beams with carbon fiber laminate (CFL) is the CFL-concrete interfacial fatigue properties. Fatigue and fracture mechanics method is used to develop a calculation formula of stress intensity factor (SIF) for the interfacial fatigue cracks. Based on theoretic analysis and a serial of fatigue tests, the propagation law and the predicting method of the interfacial fatigue cracks are discussed. The main results show that: 1) the SIF’s value, a determined factor of the fatigue crack propagation life, is a direct ratio to that of CFL strain. When the CFL strain range is below the value of a “strain threshold”, the interfacial fatigue cracks will not propagate; 2) the process for the initiation and stable or unstable growth of the interfacial cracks is observed. The interfacial fatigue crack propagation life of the RC beams strengthened with CFL under cyclic bending loads can be precisely predicted by using a similar approach as Paris formula.